This is included in all custody orders in our state:
36-6-110. Rights of non-custodial parents. —
(a) Except when the juvenile court or other appropriate court finds it not to be in the best interests of the affected child, upon petition by a non-custodial, biological parent for whom parental rights have not been terminated, the court shall grant the following parental rights:
(1) The right to unimpeded telephone conversations with the child at least twice a week at reasonable times and for reasonable durations;
To me "unimpeded" means "not interferred with". Check your orders and/or state law. If they include similar rules, then point it out to
BM, then simply hang up the phone. Or suggest that from now on, you will be recording her when she calls you to "discuss" what has been said to son. That ought to stop her; it works in our case, anyway.